【作者】 何博；

【导师】 孙宝德；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2008， 博士

【摘要】 α+β型高温钛合金以其优秀的比强度和600°C以下的良好高温表现,成为航空航天领域最广泛应用的材料之一。但两类问题的存在阻碍了其高温下力学性能的发挥:1、高温、腐蚀性的外部环境会导致钛合金表面氧化、近表面脆化;2、高温下钛合金组织稳定性下降,长时间热暴露后析出脆性有序相,降低钛合金室温塑性和断裂韧性。在钛合金表面沉积兼具抗氧化、隔热性能的热障涂层(Thermal Barrier Coating, TBC)可以同时解决上述两类问题,拓宽传统钛合金的服役温度和范围,提升效率、节约能源。本论文首先研究了钛合金表面以NiCoCrAlY为粘结层(Bond Coating, BC),8wt.%Y2O3部分稳定的ZrO2(8YSZ)为陶瓷隔热层的传统双层热障涂层的高温行为及失效机理。在这基础上,本论文提出了一种适合钛合金的新型的抗氧化粘结层材料TiAlAg合金及其高效的热喷涂制备方法。最后,本研究在TiAlAg表面使用电子束物理气相沉积(Electron Beam-Physical Vapour Deposition, EB-PVD)沉积了8YSZ陶瓷层,构成了以TiAlAg为粘结层、8YSZ为隔热层的适合钛合金基体的新型热障涂层,并观察和研究了其高温行为和失效机理。论文中使用了改进的ASTM定向拉拔、热循环和等温静置等实验方法以及光学显微镜(OM)、扫描电镜(SEM)、X-射线衍射仪(XRD)、显微压痕等分析手段,配合有限元模拟技术,系统的研究了传统和新型热障涂层的高温行为规律和失效机理。本研究主要内容和结论如下:一、分别用EB-PVD和超音速火焰喷涂(High Velocity Oxyfuel Spraying, HVOF)在钛合金表面沉积NiCoCrAlY并统一以EB-PVD在表面沉积8YSZ陶瓷层,制备了传统热障涂层并研究了其高温行为:1.钛合金表面EB-PVD沉积的NiCoCrAlY致密均匀,与基体结合紧密,“阴影效应”导致其表面EB-PVD陶瓷层组织细密,硬度高,力学性能好,因而系统热循环性能较好,800°C实验35min×220后表面基本观察不到宏观裂纹,但等温静置性能由于过早形成有害的扩散层而表现较差,800°C寿命少于24h;而HVOF沉积的NiCoCrAlY疏密不均,与基体结合界面充满微观缺陷,其上层EB-PVD陶瓷层晶粒粗大,柱状晶间结合疏松,显微硬度较低,力学性能差,因而系统热循环性能较差而等温静置性能由于缺陷对于有害扩散的阻碍而表现较好,800°C寿命大于36h。2. Ni基粘结层与Ti基基体间化学性能不匹配是钛合金表面传统热障涂层高温下快速失效的本质原因:高温下粘结层内Ni、Co等元素向基体内部急剧扩散,短时期内生成150~300μm厚的扩散层。扩散层主要物质为Ti2Ni(2Ti+Ni Ti2Ni),该相室温硬度较高(700~850HV)而高温力学性能较差。由于NiCoCrAlY与基体间线膨胀系数(Coefficient of Thermal Expansion, CTE)相差悬殊,室温时系统最大径向拉应力高达~750MPa,轴向拉应力达~154MPa,高温时径向与轴向拉应力高达20~40MPa。在热应力作用下,系统粘结层产生纵向裂纹,扩散层产生横向裂纹,导致基体直接暴露于外界环境而失效。二、通过低压等离子喷涂(Low Pressure Plasma Spray, LPPS)和惰性气体保护预处理技术在钛合金表面原位制备Ti-Al-Ag三元合金粘结层,并研究其抗氧化性能:1.将市售γ-TiAl(粒径~75μm)和Ag粉(~50μm)混合,湿法球磨获得γ-TiAl粒径为0.1~10μm,Ag粒径~10μm的混合粉体。通过低压等离子喷涂将喷雾干燥法获得的热喷涂用混合粉体沉积于基体,试样于真空热处理炉由惰性气体Ar保护,820°C热处理5h后获得单质Ag溶于γ-TiAl基体内形成的Ti-Al-Ag三元涂层,其内检测到明显Z相(Al3Ti5O2)。2. 750~800°C氧化长达100h,三元涂层外表面形成以α-Al2O3为基的氧化膜,表面局部突起TiO2颗粒,随着Ag的增加TiO2颗粒逐渐从穿越富Al氧化膜转变为粘附于富Al氧化膜表面,表面氧化膜内Al2O3含量约为TiO2的3倍。含4at.%Ag的涂层具有最好的抗氧化性能:700°C时100h氧化增重约为0.8mg/cm2,800°C时其100h氧化增重约为1.0mg/cm2。Ag带来的抗氧化作用可能是由于其降低了表面氧化膜中的θ-Al2O3向α-Al2O3相变所需要的能量,提高了相变速度所致。Ag的加入还增强了涂层抗剥落能力、降低了涂层表面粗糙度、减小了氧化膜厚度,减少了涂层内的内氧化率。三、在TiAlAg涂层表面使用EB-PVD技术沉积了8YSZ陶瓷,构成了适合钛合金基体的新型热障涂层,并研究了其高温行为:1. 800~850oC高温下等温静置100h后,TGO始终保持富Al特性;定向拉拔实验断裂面随着TGO与面层的结合程度的增加从面层/粘结层界面附近,转向深入粘结层内部,最后由于TGO的生长减缓,断裂面又恢复至面层/粘结层界面附近;热循环导致在陶瓷层表面萌生垂直于热障涂层界面的纵向裂纹,但由于新型粘结层与基体的较好匹配性,裂纹扩展至粘结层即不再前进,使得粘结层与基体的界面成为系统中最为稳定的部位。2.新型热障涂层的高温失效源于TGO的热喷涂本征缺陷,高温长期服役中的热应力又进一步降低了其致密度,由于外界气氛的长期渗透,在粘结层次表面富Ti区或更深部位生成低强度的Ti/Al混合氧化膜,一旦受到面外拉应力后即在粘结层近面层/粘结层界面处产生横向层状分离,导致了粘结层的失效。更多还原

【Abstract】 Titanium alloy, especiallyα+βtype, due to its excellent specific strength and acceptable thermal tolerance at temperatures under 600oC, is one of the most popular materials used in field of aviation and aerospace. However, its further application in the occasion used for higher temperature and reliability is hindered by two problems. The first is that the outer environment may consume the facial material, and embrittle the near-surface material. The second is that the long exposure at high temperatures may dissolve out some brittle phase, like Ti3X or silicide etc., bring down the plasticity and fracture toughness, and influence the fatigue property of the titanium-alloy. Preparation of a thermal barrier coating (TBC), which is both oxidation- and heat-resistant, on the surface of titanium-alloy supplies an attractive solution for these two problems, so that to expand the service occasion of the Ti-base materials, and raise efficiency and save resources.In this study, we firstly deposited a traditional thermal barrier coating, in which NiCoCrAlY alloy is the bond coating (BC) and 8wt.% yttria-stabilized zirconia (8YSZ) is the thermal-isolation coating, onto the surface of a titanium-alloy TC11. In the base of the knowledge to the thermal behavior and failure mechanisms of that system, we proposed a new method to prepare an oxidation-resistant ternary Ti-Al-Ag alloy coating onto the substrate as a novel BC. Finally, Electron Beam-Physical Vapor Deposition (EB-PVD) was used to deposit 8YSZ onto the ternary BC pre-treated, to construct a novel thermal barrier coating suitable for titanium alloy substrate. The modified ASTM directional pulling, micro indentation, thermal cycling and isothermal exposure tests were used to observe the thermal behavior and study the failure mechanisms. Optical microscopy (OM), SEM, XRD were used as analyses methods, assisted with the finite-element simulation. The main work and conclusions in this study are as the following:EB-PVD and HVOF (High Velocity Oxyfuel Spraying) were used to deposit NiCoCrAlY onto the titanium-alloy substrate and the top ceramic coating was deposited sequentially by EB-PVD in one batch to construct a traditional thermal barrier coating onto the titanium-alloy. TBC with the different properties of the BC were studied:NiCoCrAlY deposited by EB-PVD is denser and more homogeneous, and the bonding to the substrate is metallurgical bonding. Due to“shadow effect”, a slim-clustered upper 8YSZ, with good mechanical properties is achieved. This kind of TBC shows better thermally cycling performance at 800oC (longer than 35min× 220), but poor isothermal exposure durability (shorter than 24h). By using HVOF instead of EB-PVD for the preparation of BC, the inhomogeneity and porosity of the BC is increased, resulting in a looser 8YSZ coating having coarse grains and low hardness. The isothermal durability of TBC, however, is extended to longer than 36h at 800oC, but its thermal cycling durability is worsened.Chemical mismatch between the Ni-based BC and the Ti-based substrate is the most essential factor degreading the traditional TBC of the Ti-alloy. It’s very easy to diffuse of Ni, Co from BC to the substrate to form a 150~300μm inter-diffusion layer at the substrate near the BC. The main product is Ti2Ni which is very hard at room-temperature but very soft at high temperature near 800oC. Because the coefficient of thermal expansion (CTE) mismatch between the BC and the substrate is great, ~750MPa radial tensile stress and ~154MPa axial tensile stress arise in the system when the sample is cooled down to the room temperature after the preparation. Similarly, tensile stress of about 20~40MPa is formed when the sample is reheated to 800oC. As a result, the NiCoCrAlY BC is vertically cracked and the inter-diffusion layer is horizontally cracked during the thermal service, and the substrate is exposed into the atmosphere and failed.A novel Ti-Al-Ag ternary alloy coating was designed and in-situ prepared by low pressure plasma spray (LPPS) assisted with a sequential inert-gas-protected treatment, and its oxidation-resistance was identified and studied.The commercialγ-TiAl and Ag powders (~75 and 50μm, respectively) are used as the raw materials, and the wet-ball-milling is employed as the mixing and crashing technique. Twenty four hours later the dimension of theγ-TiAl and Ag in the mixture powder are 0.1-10μm and ~10μm, respectively. The mixture powder is spray-dried into the powder suitable for thermal spraying. Then it is deposited onto the surface of the substrate, following by the treatment under the protection of Ar with the pressure of ~1atm. at 820oC for ~5h. It is found that the silver is dissolved into the matrix of theγ-TiAl and a ternary coating is formed with good tightness. The Z phase (Al3Ti5O2) arises obviously in the ternary coating.After oxidized at 700-800oC for 100h, the oxide scale of ternary coating is basically theα-Al2O3 with some local prominence of the TiO2 particles. With the addition of the Ag, the TiO2 particle is changed from subscale-connected to the facial-only above the Al-rich scale, so that the compactness of the scale is higher. The content of the Al2O3 in facial oxide scale is about 3 times of the TiO2. The ternary alloy coating with 4at.% Ag shows the best oxidation-resistance with the 100h-mass-gain of ~0.8mg/cm2 at 700oC and 1.0mg/cm2 at 800oC. It is believed that the addition of the silver decreases the of phase-change energy of the Al2O3, so that increases its velocity of the phase-change fromθtoα. The addition of the Ag is also helpful to elevate the delamination-resistance, smooth the surface of the coating, decrease the inner oxidation, thin the oxide scale of the coating.A novel TBC suitable for titanium-alloy substrate was prepared by the deposition of the 8YSZ onto the oxidation-resistant TiAlAg coating by EB-PVD technology, its thermal behavior and failure mechanisms were studied:After oxidized at 800oC for 100h, the thermal grown oxide (TGO) between the ceramics and ternary coating is still Al-rich. The cracking interface of the isothermal exposed sample resulted from the modified ASTM directly pulling is changed from the location near the TGO/8YSZ interface to the deeper place into the BC, then turn back to the TGO/8YSZ interface, reflecting the changing of the bonding extent between the TGO and the 8YSZ and the growth of the TGO with the isothermal exposure. The thermal cycling vertically cracks the 8YSZ, but the propagation of the cracks are stopped at the 8YSZ/TiAlAg interface because of the better match of the system, as a result the BC/substrate interface is turned into a most stable location of the thermal barrier system.The final failure of the novel TBC should be ascribed to two factors. One is the intrinsic defects on the surface of the TGO induced by the thermal spraying, the other is thermal stresses arise with the severe service environment. The permeation of the outer atmosphere through these defects is slow enough without the presence of the thermal stresses, but the compactness of the TGO is worsened during the thermal cycling. The mixed oxides of Ti and Al are formed at the subscale and deeper zone during the long-term permeation of the outer atmosphere through the enlarged defects at high temperature, therefore, once the TBC is under the out-of-plane tensile stress, it may be failed by the separation around the 8YSZ/TiAlAg interface.更多还原